What is the best way to get 1.34 volts, low current to replace Hg Batt

On a sunny day (Sat, 11 Sep 2010 13:23:18 -0400) it happened "tm" wrote in :

The dropout voltage of a LM317 is between 1.25 V at 100 °C, and 1.75 V at -50 °C, at 50 mA. So that would do. Some trimpot to set the 1.34 V.

The tempco is probably lower, too...

You won't get any bonus points for leakage though. LM317 is 1mA or something, so your LiIon will bleed down fairly quickly (~weeks?).

There are precision low current references that run at microamperes. If you can find an adjustable one, you can get what you need. Else, use a CMOS op-amp and large divider resistors to set gain.

Tim

A couple of years ago I bought a couple of Hg battery replacements for cameras. Basically they're the size of the original battery and take a 1.5V buttons cell, with an LDO of some sort to drop the voltage appropriately. I don't remember the specifics.

Will it be running continuously? Then you might use one of those micropower voltage regulator chips, LT3008 or something like that.

John

"tm" wrote in news:i6gdlm$1lp7$ snipped-for-privacy@adenine.netfront.net:

why not keep it simple and run it from a pair of AA alkaline cells? No need to recharge,no expensive batteries.

That may be the "MR-9 battery adapter", although there may have been others available. A Google search found it. One source appears to be:

I don't know if the MR-9 will meet Tom's needs. It uses a 386 silver oxide cell.

Fred

Sure. Buy another mercury battery. They have much lower noise than semiconductor regulators, better output stability than other battery types, and some (small) types are still available, last time I heard, for compatibility with old gear.

An alkaline battery can make a nice 0.5 Hz triangle wave generator, because there's a gas discharge and venting occurs. If you really want to get the precision, accuracy, and low impedance of a mercury battery, resign yourself to paying for proper disposal.

Yep, that's it. The PX625 was what I needed to emulate (Canon camera).

I did see that one plus one like it in England for about $35 or so that was for the PX-1.

The cell is just used for the Ohms measurements and is switched off when not in use.

Anyway, thanks guys for the suggestions.

tm

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Why not use a zinc-air cell?

Compare the discharge curves on page 3:

--Winston

Just out of interest, these guys sell a drop in voltage reducing adaptor to do what you want:

AIUI, they have a very poor shelf life and are quite expensive, given the shelf life.

Zinc-air cells are oxygen pressure sensitive. Every breeze or exhaled breath will wiggle the needle on the meter.

At what time constant?

I see that it takes from 1 to 3 minutes for the battery to begin converting power after one removes the tab.

Is it possible that tiny momentary variations in oxygen concentration would 'average out' acceptably?

My Google-fu wasn't up to the task of answering that question.

--Winston

Thank you. That's a link worth saving.

FWIW, I was looking at an old Babani book recently where it calibrates a

;-)

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For a nearby bunch of people breathing out diminished O2, it'll be a few minutes (for diffusion). For a wind gust that lowers the local air pressure, only a few milliseconds. And for local barometric changes, there will be corresponding shifts in the average output voltage from day to day.

Heaven only knows what effect ozone or laboratory gasses might have.

If the requirement is just for a stable reference voltage, there are stabilized ICs that can handle the task. If you need low noise, and low output impedance at a broad frequency range, and good voltage stability, it's hard to beat a mercury cell, even just the 'regular' ones, not the Weston cells.

The camera-battery solutions are not reference-grade sources.

Sorry, link & site...

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Try again?

Works for me.

--Winston